TRACKING WILDLIFE BY SATELLITE 



31 



Alaska, facility, we have evaluated a LUT built by Te- 

 lonics, Inc., since September 1986. The LUT receives data 

 from the satellites' VHP transmission. The LUT consists 

 of two IBM-compatible computers: an XT model, which 

 runs a satellite prediction program that shows where the 

 satellites are at all times and characteristics of satellite 

 overpasses; and an AT model, which operates the tracking 

 antenna, receives and processes the VHP signal contain- 

 ing Argos messages, estimates PTT locations, and pro- 

 duces a report following the overpass. 



The LUT points a 4-m, two-beam yagi antenna toward 

 the satellite while it is above the horizon. The VHP signal 

 containing the Argos data is received, decoded, and stored 

 for later analysis. During each overpass, the LUT displays 

 the strength and quality of each incoming signal, the posi- 

 tion of the antenna, and the identification of PTT's for 

 which messages are being received. Doppler data from 

 reference PTT's placed at known locations are used to 

 determine the position of the satellite in its orbit more 

 precisely. After the overpass, the LUT uses satellite and 

 Doppler data, received from the reference PTT's during 

 the overpass, to estimate PTT locations. 



A LUT cannot provide as many location estimates and 

 sensor messages as standard Argos processing can. For 

 data to be received by a LUT, the satellite must view both 

 the PTT and LUT simultaneously. Argos uses tape re- 

 corders on the satellite to store messages for playback to 

 ground receiving stations; therefore, to receive data, only 

 the PTT needs to be within view of the satellite. Further- 

 more, radio interference near the horizon from VHP 

 sources, especially in metropolitan areas, may reduce the 

 number and quality of messages received by a LUT, par- 

 ticularly those with omnidirectional antennas. Addi- 

 tionally, the software used to estimate locations is propri- 

 etary and differs among LUT manufacturers and between 

 LUT's and Argos. Therefore, even when the received data 

 are identical, estimated locations may not be. 



Performance 



Reliability 



During December 1987, we compared the number of 

 messages received by our LUT from 18 PTT's deployed 

 on caribou in northern Alaska and Yukon Territory with 

 the number received using standard Argos processing. 

 The caribou ranged 394-633 km from Fairbanks during 

 the experiment (mean = 546 km). For each PTT, we calcu- 

 lated ( 1 ) the number of overpasses during which at least 

 one message was received by the LUT or Argos, (2) the 

 number of locations estimated, and (3) the total number of 

 messages received. The mean number of overpasses for 



which the LUT received data from these PTT's was 79% 

 (min.-max., 69-85%) of that for Argos. The mean number 

 of locations estimated by the LUT was 50% (min.-max., 

 24-72%) of that calculated by Argos. The LUT recorded a 

 mean of 56% (min.-max., 41-66%) of the messages re- 

 ceived by Argos for these 18 PTT's. 



Two important factors that contributed to the lower 

 quantity of data and locations provided by the LUT were 

 signal interference and the lack of adequate Doppler data 

 from reference platforms for some overpasses. Our LUT 

 was located near a major communications facility and a 

 television station; signal reception was blocked whenever 

 the antenna pointed toward these sources of radio inter- 

 ference. Primarily because of radio interference and signal 

 blockage by hills and buildings, the LUT calculated only 6 

 locations for overpasses where the maximum satellite ele- 

 vation was < 30, compared to 236 locations calculated by 

 Argos during the same experiment. We had only a single 

 reference PTT located in Alaska, and lack of sufficient 

 data from this PTT prevented the LUT from calculating 

 locations for 729 of 2,265 overpasses (32%) during the 

 December 1987 experiment. The lack of reference data 

 may not be a problem for LUT's located in the contiguous 

 United States or other locations where several reference 

 platforms can be placed within view of the satellite. Tel- 

 onics, Inc., is now testing a new LUT system with a more 

 expensive and sophisticated tracking antenna to reduce or 

 eliminate interference from other radio sources. 



Precision 



The precision of locations calculated by the LUT was 

 determined in March 1988 using nine PTT's placed at 

 known locations near Fairbanks. PTT's were placed 

 at elevations of 152, 708, and 902 m (three at each 

 site); however, an elevation of m was used in the 

 location calculations to enable comparisons of loca- 

 tion accuracy with standard Argos processing. Loca- 

 tions estimated by the LUT (n = 93) had a mean error 

 of 12.3 km. (For comparison, the mean error for 354 

 locations estimated by Argos was 1 .4 km.) Fifty-six per- 

 cent of the LUT locations were within 5 km of the true 

 location. When overpasses with a maximum satellite ele- 

 vation exceeding 70 were excluded, the mean error of 

 locations estimated by the LUT fell to 5.7 km, and the 

 percentage of locations within 5 km of the true PTT loca- 

 tion rose to 68%. Argos rarely calculates locations for 

 overpasses greater than 70 because of poor location 

 accuracy. 



Precision of locations dropped off markedly when 

 PTT's were far from the nearest reference platform. The 

 transmitter at Nome, Alaska, had been used as a reference 

 platform for our LUT and helped to calibrate locations in 



